The leak one fools more people than the animals do, and the tell is wet sticky ground or a sticky pole with ants gathering, plus emptying faster on hot days or in direct sun. A saucer-style feeder does not rely on a vacuum and rarely drips.
If you want certainty rather than deduction, point a cheap trail camera or an old phone at it overnight. One night settles it, and watching whoever turns up is genuinely good value.